The purpose of this study is to better understand the pathogenesis of atherosclerosis. Two of the earliest events in atherogenesis are the accumulation of low density lipoprotein (LDL) and macrophages in the arterial intima. Blood monocytes enter the intima, become macrophages, accumulate lipid and become foam cells. Monocyte adhesion to endothelium is necessary for transmigration into the vessel wall. Monocyte adhesion depends upon the level of expression of adhesion receptors by endothelial cells and monocytes as well as the fluid forces acting on monocytes. Localization of early lesions to vessel branches may result from a hemodynamic environment which does not cause cell detachment and alters endothelial cell function to promote monocyte adhesion. Further, local oxidation of LDL within the arterial wall may activate endothelial cells. Arterial fluid dynamics may affect the transport of monocytes and LDL to endothelium, further influencing the localization of the disease. Cell culture studies will be performed to test the following two hypotheses: 1) arterial fluid dynamics and local oxidation of LDL activate the endothelium to express receptors for monocytes, and 2) localization of monocyte adhesion to vessel branch points represents a balance between hydrodynamic and adhesive interactions.The specific aims related to this work are 1) biophysical characterization of monocyte interactions with endothelial cells activated with minimally modified LDL (mmLDL), 2) effect of flow field upon the expression of adhesion molecules by endothelial cells, and 3) monocyte adhesion to and rolling on activated endothelial cells in a region of flow recirculation. For aim #1, micropipet and flow experiments will be performed to characterize kinetics of U937 cell and monocyte attachment and detachment from human endothelial cells activated with TNFa or mmLDL, and L cells which express a single adhesion molecule. The role of selectin and integrins in rolling and firm arrest will be determined by measuring rolling velocities in the presence of antibodies which block receptor-ligand binding. Experiments will be performed with and without red cells present. We will obtain bond lifetimes and kinetic constants will be compared with estimated values from micropipet and flow experiments. For aim #2, endothelial cells, with or without mmLDL treatment, will be exposed to either fully developed laminar with parallel streamlines or recirculating laminar flow at shear stresses between 5 and 20 dyne cm-2 for 24 h. Adhesion molecule expression will be determined by relative quantitation of immuno- fluorescence. In some experiments cells will be exposed to mmLDL and TNFa. Monocyte rolling and firm arrest will be measured following exposure of endothelium to flow. Results from these studies will be used to evaluate the extent to which the local fluid dynamics influences activation of endothelium. For aim #3, we will use a sudden expansion flow chamber to examine the effect of curved streamlines, fluid forces, and torque upon monocyte cell transport and attachment to endothelium. This chamber creates a region of flow recirculation and simulates flows occurring at vessel branches. The stresses and monocyte trajectories will be calculated using computational fluid dynamics. We will measure monocyte rolling velocities and firm arrests at various locations in the flow field. We will evaluate the effect of red cells on adhesion and rolling.